I've heard that EV trucks are mostly unibodies given the structure they need to protect and support the battery underneath. But I think the F-150 lightning is simple body on frame (as you said, although I think you are confused that this doesn't exist yet?), and the new Silverado EV will be neither unibody nor body on frame, whatever that means.
Unibody and body on frame have different pros and cons. Most people buying a truck want a unibody. The F150 lightening is only available in the crew cab version - you can get cars with as much cargo space. If you are actually hauling heavy things, or towing a trailer then a frame is strong and better able to handle that. However if you are only hauling people with some light luggage then a unibody is plenty strong and betters resists twisting forces making for a nicer ride.